We have been using the 68LC302 for decades without major problems. We have designed a new product using the same circuitry around the 68LC302 as before but we can't get it to serial boot. PA5, PA7, PA12 & DISCPU are all low for serial boot but it won't echo data sent to SCC1. The clock is running correctly but the CPU resets every ~4.2uS, with internal activity seen on /DTACK synchronous to this. It isn't the MAX691A reset chip, the reset pulses are from the CPU. We have built 2 prototypes and both behave the same. We've been checking this from every angle for days. Help!